1. Field of the Invention
The present invention relates to a vehicle steering system and more particularly to a hydraulic power assisted vehicle steering system. More specifically, the present invention pertains to a hydraulic power assisted vehicle steering system having a self centering mechanism.
2. Description of the prior Art
It is commonly recognized that conventional vehicle steering systems have a self centering nature due to the caster angles of front wheel arrangements so that the steering mechanism tend to assume a neutral position when the steering force on the steering wheel is relieved. In case of a hydraulic power assisted steering system, however, flow restrictions in the hydraulic system function to resist such self centering movements so that it becomes necessary to apply a manual force to return the steering wheel to the neutral position.
The U.S. Pat. No. 3,373,631 proposes a self centering mechanism for a hydraulic power assisted steering system. According to the proposal, there is provided a cam which is adapted to be rotated in response to a steering movement of the steering shaft. A cam follower roller is provided to be in contact with the cam and forced by a hydraulic piston into engagement with the cam. Thus, the cam is hydraulically forced toward the neutral position whenever the manual steering force on the steering shaft is relieved. In the U.S. Pat. No. 3,426,612, there is also disclosed a similar self centering mechanism for a hydraulic power assisted vehicle steering system. It should be noted that the self centering mechanism as proposed by the above patents are disadvantageous in that complicated cam mechanisms are required.
In Japanese patent application 58-92887 filed on May 26, 1983 and disclosed for public inspection on Dec. 11, 1984 under the disclosure number 59-220455, there is disclosed a hydraulic power assisted vehicle steering system which includes a hydraulic circuit for switching the supply of hydraulic liquid to the power cylinder when the manual steering force on the steering wheel is relieved. The self centering mechanism as proposed by the Japanese patent application is also disadvantageous in that complicated hydraulic circuits are required.